Re: doing a string comparison with an extra level of variable reference?

[John Graham-Cumming]

On Fri, 2005-01-28 at 08:21, Robert P. J. Day wrote:
>   ${SUBDIRS}:
>         @echo "My target is address@hidden"
>         @echo "My makefile variable contains address@hidden"
>   ifneq "address@hidden" ""
>         @echo "Yup, that variable is set."
>   endif
> 
> i want to call this with something like:
> 
>   $ make d2_MAKEFILE=d2make d2
> 
> and be able to test whether the corresponding variable is set within
> the rule.

That doesn't work because the ifneq is handled at Makefile parse time
and hence $@ is unset and if ends up doing

    ifneq "${_MAKEFILE}" ""

which assuming you haven't set _MAKEFILE somewhere is going to fail. 
Since $@ is only known at rule run time and preprocessor stuff like
ifneq is done at parse time there's no way to mix the two.

You could use a $(if) instead like this:

  ${SUBDIRS}:
        @echo "My target is address@hidden"
        @echo "My makefile variable contains address@hidden"
        @$(if address@hidden,echo "Yup that variable is set.")

Note that I took the comma out of the "Yup..." otherwise the $(if)
interprets that , as a parameter separator.

John.
-- 
John Graham-Cumming

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